Drilling fluids are widely used for the drilling of oil and gas wells. These drilling fluids provide suppression of reservoir pressure, lubrication of the drill pipe and effect cooling of the bottom hole drill assemblies. Down-hole assemblies may contain individual components such as bits, stabilizers, measurement while drilling tools, etc. Often the down-hole assemblies contain electronic instruments that contain microprocessors that are used to collect and/or transmit data collected by various sensor arrays.
Drilling fluids generally contain a variety of elements, both desirable and undesirable, such as mud, chemicals, drill cuttings, metal shavings, etc. The particle size of these various elements varies from a few microns to several inches. Additionally, rig crews often inadvertently drop tools, gloves, rags, and other foreign or unwanted materials into the well bore. In addition, broken pump and valve parts are often dropped into the mud reclamation tanks and are carried down stream in the drill string. The unwanted and/or undesirable solids, referred to as debris, can be extremely harmful to down-hole tools containing instruments and the like. Therefore, it is desirable to filter the drilling fluid at the drill floor.
Many methods of filtering well bore fluid have been used. One such method includes placing a filter-like screen in the tubular members while the tubular members are being run into the well bore. The prior art devices presently available utilize a cylindrical or conical screen with an external retrieval neck.
It is quite beneficial to screen the drilling fluid being pumped down-hole through the annulus of the drill string at the drill floor in a manner that is supposed to eliminate any foreign debris from becoming entrained in the fluids that may plug or damage any of the down-hole tools or sensitive instruments located therein during drilling operations.
However, it has been the practice to remove such filter screens at some point to allow for passage of down hole tools though the annulus. Therefore, it has been the accepted practice to install filter screens at critical locations, such as at the entrance to the mud pump, but such filter screens have often proven inadequate and provide no protection from debris that passes though the tubing from the mud reclamation tanks after the above described screen filters have been removed. Although such mud screens have been inserted into the tubular drill string, they have the disadvantage of being cumbersome to install and difficult to remove or to clean if necessary. In many cases, the removal can only be accomplished by tripping the pipe out of the hole which, of course, becomes impossible in the event that the pipe is stuck. If left in place, the down-hole screen will provide a blockage to any tools, such as survey instruments, string shots, etc., that may be run any time during the drilling operation. Of course, the screen may eventually become plugged and severely limit the flow of fluid unless it is removed and cleaned occasionally. Down-hole type screens heretofore provided, although capable of removal, run an extreme risk that the debris collected by the screen will escape from the screen during the removal process and plug the down-hole devices meant to be protected in the first place. Therefore, there is a need for a tool joint filtering device that remains in the well annulus, adjacent a down-hole tool, which efficiently filters the drilling fluid while still allowing maximum flow-through capabilities. It is far better to retrieve a tool that has become inoperative due to a plugged filter than due to extreme damage.
In addition, since the down-hole drilling fluids are quite abrasive and are pumped at high pressures of approximately 5000 PSI, any reductions in orifice size increase velocity. Therefore, filters and strainer disks having such orifices suffer significant wear and breakage. It becomes obvious that in cases where high velocity is not required orifice size should be kept to a maximum and that more attention should be paid to the type of metal, surface preparation, and cladding.
In some cases where high velocity is an advantage, such as that disclosed by Gaylord in U.S. Pat. No. 3,831,753, wherein the strainer is comprised of a pair of stacked disks having a plurality of slots with the disks rotated in a manner whereby the slots are approximately 90 degrees apart. This arrangement provides a plurality of square holes subjected to high-pressure abrasion. In addition the strainer disk is seated within a pipe joint subsection that adds cost to the strainer. The Gaylord apparatus provides an increase in velocity and excessive wear that is undesirable in other locations in the drill string. Such is the case of filtering out damaging debris from entering sensitive instrument tool joints where breakage of the strainer disk could create the problems mentioned herein.
It should be clearly understood that, in most cases, restriction of fluid flow by a filter element should be kept to a minimum while preventing large, potentially damaging debris from entering sensitive areas.